Interpretation of role of methane in low-temperature oxidation processes of methane/n-heptane mixtures

To understand the function of methane in low-temperature oxidation processes of methane/n-heptane mixtures, mole fractions of various species in oxidation processes occurring in an atmospheric-pressure jet-stirred reactor were quantified at different methane contents over the temperature range of 500–750 K and equivalence ratios of 0.5, 1.0 and 2.0 with a residence time of 1.4 s. After examining the capability of the NUI model in reproducing the experimental data, physical and chemical roles of methane were separated from each other. Chemical kinetic characteristics behind the action of methane were revealed through analyses of reaction pathways and competition for radicals between methane and n-heptane. The result shows that the addition of methane inhibits nonlinearly the low-temperature oxidation of n-heptane. The physical effect of methane is equivalent to that of nitrogen under the high-dilution condition, and as the methane content reaches 50%, methane starts to show the chemical effect, leading to the decrease of the maximum conversion ratio of n-heptane. The reaction pathway analysis shows that under the studied conditions, the stable oxygenated species and olefins will be left, and the added methane mainly affects the reaction pathways of small-molecule intermediates with OH and HO2. At higher methane content, less ketohydroperoxides and hydroperoxyl cyclic ethers will be consumed, and the negative effect of the reaction R46 CH4 + OH <=> CH3 + H2O on the consumption of n-heptane strengthens gradually. In addition, it is found that contributions of elementary reactions related to C7 species to generation and consumption of OH and HO2 remain unchanged under different methane additions, and methane suppresses the low-temperature oxidation of n-heptane mainly through its subsequent reactions to compete for OH and HO2 with small-molecule intermediates.